An optical fiber conventionally consists of an optical core, whose function is to transmit and possibly to amplify an optical signal, and an optical cladding, whose function is to confine the optical signal within the core. For this purpose, the refractive indexes of the core nc and of the outer cladding ng are such that nc>ng.
For short-distance applications and for local networks, multimode fibers are frequently used. The core of a multimode fiber generally has a diameter of approximately 50 microns (μm), compared with approximately 9 μm for the core of a single mode fiber. Therefore, for a given wavelength, several optical modes propagate simultaneously along the fiber, conveying the same information. The bandwidth is directly related to the group velocity of the optical modes propagating in the multimode core of the fiber. To guarantee a large bandwidth, the group velocities of all the modes must be identical, i.e., the intermodal dispersion must be zero or at least minimized for a given wavelength. Multimode fibers have been the subject of international standardization under standard ITU-T G.651 (withdrawn on Aug. 16, 2008) and the improved standard ITU-T G.651.1 (approved on Jul. 29, 2007), which in particular defines criteria for modal bandwidth-length product for overfilled launch [minimum at 850 nm of 500 MHz·km and minimum at 1300 nm of 500 MHz·km], numerical aperture [0.20±0.015], and core diameter [50±3 μm], required for compatibility between fibers. The recommended multimode fiber supports the cost-effective use of 1 Gbit/s Ethernet systems over link lengths up to 550 meters, usually based upon the use of 850-nm transceivers. The ITU-T G.651 and ITU-T G.651.1 recommendations are hereby incorporated by reference.
For optical fibers, the refractive index profile is generally presented as a graph plotting the refractive index against the fiber radius. Conventionally the distance r to the center of the fiber is shown along the abscissa, and the difference between the refractive index and the refractive index of the fiber cladding is shown along the ordinate axis. The refractive index profile is therefore referred to as a “step,” “trapezoidal,” “triangular,” or “alpha” profile for graphs having the respective shapes of a step, trapezoid, triangle, or gradient. These curves are generally representative of the theoretical or set profile of the fiber, the manufacturing stresses of the fiber possibly leading to a slightly different profile.
In step-index fibers having a “step” core profile, the different modes propagate at different speeds along the fiber. This causes spreading of the light pulse, which may become comparable to the spacing between pulses, possibly leading to an unacceptable error rate. To reduce intermodal dispersion in a multimode fiber, it has been proposed to manufacture graded-index fibers with an “alpha” core profile. Such fibers have been used for many years, and their characteristics have been described in particular in the following publications: “Multimode Theory of Graded-Core Fibres” by D. Gloge et al., Bell System Technical Journal, 1973, pp. 1563-1578, and “Comprehensive Theory of Dispersion in Graded-Index Optical Fibers” by G. Yabre, Journal of Lightwave Technology, February 2000, vol. 18, No. 2, pp. 166-177. Each of these publications is hereby incorporated by reference in its entirety.
A graded-index or alpha profile can be defined as the relation between the value n of the refractive index at one point in relation to the distance r from this point to the center of the fiber:
  n  =            n      1        ⁢                  1        -                  2          ⁢                                    Δ              ⁡                              (                                  r                  a                                )                                      a                              
where α≧1; (α→∞ corresponding to a step index); n1, the maximum refractive index value of the multimode core; a, the radius of the multimode core; and
  Δ  =            (                        n          1          2                -                  n          0          2                    )              2      ⁢              n        1        2            
where n0 is the minimum refractive index value of the multimode core, generally corresponding to the refractive index value of the cladding (which is most often of silica).
A graded-index multimode fiber therefore has a core profile with radial symmetry such that, along any radial direction, the value of the refractive index decreases continuously from the center of the fiber towards its periphery.
When a multimode light signal propagates in the graded-index core, the different modes see a different propagation medium, which has a different effect on their speed of propagation. By adjusting the value of parameter a, it is therefore possible to obtain a group speed that is practically equal for all the modes and hence a reduced intermodal dispersion.
This adjustment of parameter a theoretically allows a group speed to be obtained that is practically equal for all the modes, but the profile of the multimode fiber actually manufactured comprises a graded-index central core surrounded by an outer cladding with constant refractive index. Therefore, the core of the multimode fiber never corresponds to a perfect alpha profile since the interface with the outer cladding interrupts this alpha profile. Modes of highest order will therefore be accelerated relative to the lower order modes, and the bandwidth will thereby be reduced. This phenomenon, known under the expression “cladding effect,” is seen when measuring modal dispersion delay, which shows multiple response pulses when the majority of excited modes are the highest order modes.
This problem has already been identified. To solve this problem, it has been proposed to insert an intermediate depressed cladding between the graded-index core and the outer cladding. The alpha profile of the core is therefore extended into a depressed cladding. This solution is described in particular in International Publication No. WO 2006/010798, (and its counterpart U.S. Pat. No. 7,646,955), each of which is hereby incorporated by reference in its entirety, in which the core is formed by controlled germanium doping (up dopant) and the depressed cladding is obtained by doping with fluorine (down dopant).
Such a solution requires perfect control over the dopant concentrations to guarantee continuity of the alpha profile, in particular at the core/depressed cladding boundary. In practice, it is almost impossible to ensure continuity of the slopes, i.e., a smooth slope profile without sudden changes in the slope of refractive index profiles of the core (doped with germanium) and of the depressed cladding (doped with fluorine). This rupture or discontinuity of slope in the alpha profile caused by a switch of an up dopant for a down dopant causes perturbations for transmission of the highest order modes and consequently a reduction in the bandwidth.
U.S. Pat. No. 4,339,174, which is hereby incorporated by reference in its entirety, describes a multimode fiber with a core having a graded-index profile and a rupture point. Such a rupture point can cause perturbations, however, in fast bit rate transmissions.
U.S. Pat. Nos. 4,229,070 and 4,230,396, each of which is hereby incorporated by reference in its entirety, describe multimode fibers having profiles obtained by co-doping in the core and in the depressed cladding surrounding the core.
For applications of fast bit rate Ethernet networks over long-haul distances, in particular for bit rates of 10 GbE (10 Gb/s) or higher, over more than 300 meters (respectively 550 meters), the guaranteed effective bandwidth must be 2000 MHz·km or greater (respectively 4700 MHz·km). Standard TIA-492AAAC-A standardizes the performance levels required for fast rate multimode fibers having a diameter of 50 μm. For multimode fibers, the bandwidth depends on the source used. The effective bandwidth (called EMB for Effective Modal Bandwidth) corresponds to the smallest bandwidth for all the EMBs of Source-Fiber pairs for all standardized sources in 10-GbE applications. In order to attain the performance levels laid down by standards, it is necessary to eliminate any discontinuity or rupture of the slope of the alpha profile that could occur at the interface between the core and the depressed cladding.